Re: HV probe where did I go wrong

Kevin Wood G7BCS

Isn't the problem that, if the 'scope is in AC mode, the 1M input
impedance is behind the coupling capacitor so there is no DC path to
ground. If the probe is presented with a large DC voltage, then the
coupling capacitor can charge up until something breaks down, at which
point the energy might well end up zapping the front end.

Certainly, a quick look at a 7A26 plugin diagram (easiest to hand) shows
that there is no 1M path to ground for DC on the AC setting.


I was under assumption that the 1 meg input of the scope was the ground
end of the divider.
It measures 1 meg (less on the very lowest ranges)
I must not be understanding input attenuator and input impedance

From: mailto:TekScopes@...
Sent: Monday, June 22, 2015 8:28 AM
To: TekScopes@...
Subject: [TekScopes] Re: HV probe where did I go wrong

Maybe I don't understand the issue correctly, but it seems to me that the
HV probes, when connected to the scope input, will have the correct
termination & attenuation without using any external resistors. If you
used an external resistor in series with the probe output, then that
explains why the input got zapped, because no division of the HV is
happening with the voltage divider being open-ended.
HankC, Boston

Posted by: "Paul Kraemer" <elespe@...>


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